Fix CSS classes for the post editor iframe body.

packages/block-editor/src/components/iframe/index.js

@@ -131,27 +131,30 @@ function Iframe( {
 		function preventFileDropDefault( event ) {
 			event.preventDefault();
 		}
+
+		const { ownerDocument } = node;
+
+		// Ideally ALL classes that are added through get_body_class should
+		// be added in the editor too, which we'll somehow have to get from
+		// the server in the future (which will run the PHP filters).
+		setBodyClasses(
+			Array.from( ownerDocument.body.classList ).filter(
+				( name ) =>
+					name.startsWith( 'admin-color-' ) ||
+					name.startsWith( 'post-type-' ) ||
+					name === 'wp-embed-responsive'
+			)
+		);
+
 		function onLoad() {
-			const { contentDocument, ownerDocument } = node;
+			const { contentDocument } = node;
 			const { documentElement } = contentDocument;
 			iFrameDocument = contentDocument;
 
 			documentElement.classList.add( 'block-editor-iframe__html' );
 
 			clearerRef( documentElement );
 
-			// Ideally ALL classes that are added through get_body_class should
-			// be added in the editor too, which we'll somehow have to get from
-			// the server in the future (which will run the PHP filters).
-			setBodyClasses(
-				Array.from( ownerDocument.body.classList ).filter(
-					( name ) =>
-						name.startsWith( 'admin-color-' ) ||
-						name.startsWith( 'post-type-' ) ||
-						name === 'wp-embed-responsive'
-				)
-			);
-
 			contentDocument.dir = ownerDocument.dir;
 
 			for ( const compatStyle of getCompatibilityStyles() ) {